Generally, centrifuges are apparatuses in which a rotor holding a bucket, in which a sample is placed, is rotated at high speed to apply high centrifugal force to the sample, so that a high density fraction is moved radially outwards and a low density fraction is moved radially inwards, thus separating the fractions from each other. FIG. 1 is a perspective view showing a conventional automatic balancing centrifuge. As shown in FIG. 1, the automatic balancing centrifuge includes a base 110, a centrifugal motor 120 which is supported by the base 110, a rotor 130 which is coaxially coupled to the centrifugal motor 120 and has a rotor lever 160, and a bucket 140 which has a predetermined volume to contain therein a desired sample 150 to be separated into fractions. The rotor 130 is provided with an upper housing 172 having a cubic cap shape and a lower housing 174 in which one rotor lever 160 is mounted. The rotor lever 160 has a lever central body 161 which is positioned at the center of the rotor lever 160, and two rotating arms 162 which are coupled at opposite ends of the lever central body 161 by hinges 163, thus being rotatable vertically. A pressure sensor 168 is mounted on a lower portion of each of the rotating arms 162 provided at the opposite ends of the lever central body 161, so as to directly sense the pressure of the rotating arm 162. The rotating arms 162 are balanced by the support of the pressure sensors 168. Two lever guide holes 166 are provided at opposite sides on outer surfaces of the upper and lower housings 172 and 174. The rotor lever 160 is inserted through the lever guide holes 166 and is horizontally movable in the lever guide holes 166 to maintain balance. In the drawings, the reference numeral 164 denotes a stop protrusion which is provided on each inner surface of the rotating arms 162 to hold the bucket 140. The reference numeral 142 denotes a stop groove which is formed on the bucket 140 to engage with the stop protrusion 164. In this construction, when measuring values of the opposite pressure sensors 168 are different in the state in which the samples 150 are placed in the buckets 140, a lever moving motor 182 provided in the upper housing 172 is operated. Then, the rotor lever 160 is horizontally moved by the operation of the lever moving motor 182, thus becoming balanced.
According to such a conventional automatic balancing centrifuge, imbalance of samples supported by the rotor lever is sensed prior to every centrifugal operation and, thereafter, the rotor lever is moved according to the imbalance sensing result, thus automatically balancing the centrifuge. Therefore, the samples are prevented from damage due to imbalance of the rotor and, as well, the lifetime of the automatic balancing centrifuge is extended. However, the conventional automatic balancing centrifuge is problematic in that because the weight sensor such as the pressure sensors are frequently struck by the buckets containing the measurement objects, the weight sensor is easily damaged.